1. Field of the Invention
The present invention relates to boundary acoustic wave devices using boundary acoustic waves propagating along an interface between different media, and more particularly, to a boundary acoustic wave device including a multilayer structure formed by stacking at least three media.
2. Description of the Related Art
A variety of devices using boundary acoustic waves, such as resonators and band-pass filters, have been proposed. Boundary acoustic waves propagate along the interface between different media. Therefore, the packages of boundary acoustic wave devices can be simpler than those of surface acoustic wave devices using surface acoustic waves. The boundary acoustic wave device thus can be more simplified, and have reduced thickness.
A non-patent literature document (Toshio IRINO, et al., “Propagation Boundary Acoustic Waves Along a ZnO Layer Between Two Materials”, IEICE Material, Vol. 86, No. 177, US86-39, 1986, pp. 47-54), discloses a boundary acoustic wave device. The boundary acoustic wave device has a multilayer structure including a first medium of SiO2 or Si, a ZnO third medium and a SiO2 second medium stacked in that order. An IDT (interdigital transducer) is disposed along the interface of the first medium and the third medium.
The vibrational energy of boundary acoustic waves is trapped in the third medium made of ZnO in which acoustic velocity becomes low, and thus boundary acoustic waves are propagated. In this device, the IDT is made of Al.
WO98/52279 discloses a boundary acoustic wave having a multilayer structure including a first medium, a third medium and a second medium stacked in that order as in the non-patent literature document discussed above, the first medium is made of LiNbO3, the third medium is made of SiO2, and the second medium is made of SiN. An Al IDT is disposed between the first medium and the third medium.
The boundary acoustic wave devices disclosed in the two prior art references discussed above each have an IDT made of Al. In boundary acoustic wave devices using Al electrodes, the acoustic velocity of transverse waves tends to be higher, and the trapping efficiency of the vibrational energy of the boundary acoustic waves tends to be lower, in comparison with boundary acoustic wave devices using electrodes made of a metal having a higher density than Al, such as Au, Ag, or Cu.
For developing a boundary acoustic wave device, in general, it has been considered that the trapping of the vibrational energy mainly depends on the third medium in which the acoustic velocity of transverse waves is low, and nobody has thought that trapping of the vibrational energy can be achieved by appropriately selecting the material of electrodes. Accordingly, the trapping efficiency of the vibrational energy is not satisfactory, and the thicknesses of the first and second media are increased. It has been thus considered that boundary acoustic wave devices are difficult to reduce in size.
While many of the materials used as the first to third media propagating boundary acoustic waves have negative temperature coefficients of acoustic velocity (TCV), SiO2 has a positive TCV. Hence, a combination of SiO2 and a material having a negative TCV can make the TCV value zero or close to zero.
The frequency temperature coefficient TCF of the boundary acoustic wave device results from the subtraction of the linear expansion coefficient of the boundary wave propagation path from the TCV. Thus, a combination of SiO2 and another medium material can achieve a boundary acoustic wave device having a low frequency temperature coefficient TCF.
The IDT of such a known boundary acoustic wave device is made of Al, as described in the prior references mentioned above. In a structure including a SiO2 third medium and an Al IDT, the SiO2 fills the spaces between the Al strips arranged at periodic intervals of the IDT and the reflectors. The difference in density between Al and SiO2 is small, and the difference in acoustic impedance between them is also small. Accordingly, the reflection of the boundary acoustic waves from the IDT and reflectors is reduced for each Al strip.
If the reflection from each of the strips, which are electrode fingers, is reduced, a large number of electrode fingers are required in order for the reflectors to ensure a sufficient reflection coefficient. Accordingly, the reflectors are inevitably large, and the resulting boundary acoustic wave device therefore must be large.
In addition, if the reflection from the IDT is reduced for each strip, the shape factor of a longitudinally coupled resonator-type boundary acoustic wave filter or the directivity of the EWC SPUDT of a transversal boundary acoustic wave filter is degraded, for example.
In a boundary acoustic wave device having a multilayer structure of second medium/third medium/IDT/first medium, boundary acoustic waves propagate with the vibrational energy being trapped in the third medium and the IDT. If the thickness of the third medium is relatively large with the wavelength of propagating boundary waves, higher-order modes are relatively strongly excited. Therefore, the thickness of the third medium is preferably smaller than or equal to the wavelength of a single wave of the boundary acoustic waves.
If the third medium is formed by deposition, such as sputtering, it is difficult to increase the thickness of the third medium to a sufficiently larger value than the thickness of the strips of the IDT and reflectors. A third medium having a small thickness may be cracked due to the unevenness between regions having the strips and regions having no strips.